Liquid crystalline polyesters prepared from many different monomers are disclosed in the prior art. U.S. Pat. No. 3,036,990 discloses fully aromatic polyesters prepared from hydroquinone, isophthalic acid, and terephthalic acid. U.S. Pat. No. 4,169,933 discloses liquid crystal copolyesters prepared from terephthalic acid, 2,6-naphthalenedicarboxylic acid, a diacyl ester of hydroquinone and p-acyloxybenzoic acid and contains certain divalent radicals.
Prior to the present invention melt-processable all-aromatic liquid crystalline polyesters required either a hydroxy acid (unsymmetrical by nature because the two reactive groups are different), a substituted aromatic dicarboxylic acid or diol, or a kinking component, e.g., isophthalic acid or oxydibenzoic acid, in order to lower high melting points.
The solution to the problem of highly aromatic liquid crystal polyesters having a melting point so high that the polyester could not be melt processed in conventional equipment was to lower the melting point to an acceptable level by either using a substituted aromatic diol or dicarboxylic acid such as phenyl-, bromo- or chlorohydroquinone, or chloroterephthalic acid; a kinked structure such as a isopropylidene, sulfone, or keto linkage between the aromatic rings; or a meta directed or substituted aromatic dicarboxylic acid or diol, such as isophthalic acid or resorcinol; and/or a hydroxy acid monomer, such as p-hydroxybenzoic acid or 2-hydroxy-6-naphthoic acid. While these approaches were effective in lowering the melting point of the polymer, there was also a reduction in solvent resistance. Additionally, the polymers of the prior art containing the types of structures described above have much reduced heat resistance (heat deflection temperature) due to the presence of the substituents or the kinks or the hydroxy acid components.
In light of the above it would be very beneficial to be able to produce wholly aromatic melt-processable liquid crystal polyesters having the desirable combination of solvent resistance, exceptional oxidative stability and very high heat resistance.